1. Field of the Invention
The present invention is directed to a magnetic field detector for a medical device implantable in the body of a patient, having a motion-sensitive sensor to which an evaluation unit for the sensor signal is connected, an electrical coil that is secured to the sensor, and a current source that is connected to the coil and charges the coil with an electrical current for detecting a magnetic field in combination with the sensor and the evaluation unit.
2. Description of the Prior Art
A magnetic field detector disclosed in German OS 2 217 400 is composed of a magnetic switch or reed relay that is arranged in a heart pacemaker. The magnetic switch is closed by applying a magnet to the skin of the patient in whom the heart pacemaker is implanted, as a result of which a circuit in the heart pacemaker for defining the stimulation threshold of the heart is activated.
As long as only test functions of the implanted device such as, for example, identifying the stimulation threshold or a battery test, are to be triggered with the assistance of the external magnet, magnetic switches in the form of a reed relays have proven adequately reliable. Far higher demands are made on the reliability and response precision of the magnetic field detector if used to trigger (initiate) a treatment process, for example a stimulation pulse, defibrillation shock or the output of a medication dose, administered by the implanted device, or if it is used to inhibit such a treatment process that is automatically started on the basis of measured parameters of the device.
A general purpose magnetic field detector is disclosed in U.S. Pat. No. 4,887,032 wherein an electrical coil that is charged with alternating current by a current source is secured on a motion-sensitive piezoelectrical sensor. The frequency of the alternating current is matched to the resonant frequency of the sensor, so that the sensor is placed in a resonant oscillation given the presence of a magnetic field, whereby the output signal of the sensor is utilized in an evaluation unit for detecting the magnetic field.
It is an object of the present invention to optimize a known magnetic field detector of the type described immediately above for employment thereof in a medical device implantable in the body of a patient.
This object is achieved in accordance with the principles of the present invention in a magnetic field detector having the basic components of the detector of U.S. Pat. No. 4,887,032, but specially designed for implanted medical use. To this end, the current source in the magnetic field detector is connected to a novel control unit that generates a control signal for switching the current source on and off at prescribed points in time. The control signal is supplied to an evaluation unit which includes means for modifying the sensor signal with the control signal in such a way that the control signal defines the sensor signal as a detection signal for a magnetic field when the current source is switched on and defines the sensor signal as an activity signal corresponding to the physical activity of the patient when the current source is switched off.
A motion-sensitive (i.e., force-sensitive) activity sensor having evaluation circuitry for controlling the activity of a heart pacemaker is known from, for example, U.S. Pat. No. 4,428,378; however, a magnetic field detector and an activity sensor united in a single component have heretofore been unknown so that the space required in an implantable medical device employing the invention for realizing these two different sensor functions is minimized. When the current source for charging the coil with current is switched off, the sensor signal is defined by the control signal of the control unit as the activity signal corresponding to the physical activity of the patient. When the current source is switched on, by contrast, the sensor signal is indicated as the detection signal for a magnetic field.
The motion-sensitive (i.e., force-sensitive) sensor in the magnetic field detector of the invention is preferably a piezocrystal that is distinguished by a small structural size and by a high output signal.
It is necessary to be able to reliably distinguish the signal part of the sensor signal dependent on a magnetic field, that is generated by the magnetic field detector of the invention given the presence of an external magnetic field with the current charging the coil switched on, from the activity-dependent signal parts. For this purpose, it is possible to provide a defined curve or course for the current through the coil which is then embodied in the sensor signal. An individual electrical current pulse has thereby proven advantageous because the sensor signal thereby generated in the presence of an external magnetic field characteristically differs from the signals produced by mechanical impact stresses. The control signal of the control arrangement thereby defines the sensor signal as a detection signal for a magnetic field during a prescribed time interval that proceeds beyond the end of the current pulse, so that reverberations, caused by ringing of the sensor, that immediately follow the current pulse are not misinterpreted as activity of the patient. Moreover, the power consumption is especially low given the use of a single current pulse.
In this context, the evaluation unit of the magnetic field detector of the invention preferably contains a frequency discriminator and a threshold detector for monitoring the sensor signal for upward transgression of a prescribed signal threshold at a frequency corresponding to the natural frequency of the sensor. As a consequence of the individual current pulse which is supplied to the sensor given the presence of a magnetic field the motion-sensitive sensor behaves as if mechanically excited by an individual, brief mechanical impulse and then seeks to settle unimpeded with its natural frequency. The sensor signal corresponding thereto is a decaying oscillation with the natural frequency of the sensor, and thus clearly differs from other noise signals.
The reliability of the magnetic field detection can also be increased in an advantageous way by activating the frequency discriminator during the prescribed time interval when the coil is charged with the electrical current, so that the detection of a magnetic field is chronologically limited to the duration of the time interval.
For evaluating the activity signal, a signal processing circuit for the activity signal is preferably connected to the sensor, this signal processing circuit being inhibited when the coil is charged with the electrical current during the prescribed chronological duration.